Particle Acceleration in Magnetic Reconnection from the Nonrelativistic to Relativistic Regime: The Dominant Acceleration Mechanism and Formation of Power-law Energy Distributions

Magnetic reconnection is a universal plasma process that leads to sudden release of magnetic energy and strong plasma heating and nonthermal acceleration. While in situ measurement and kinetic simulations have provided much insight, the acceleration model for large-scale reconnection regions such as those in solar flares and other astrophysical environments has no consensus. We present 2D and 3D large-scale fully kinetic particle-in-cell simulations for magnetic field reconnection from the nonrelativistic to relativistic regimes. In the relativistic regime where the magnetic energy is dominant, particles are efficiently accelerated into a power-law distribution with index approaches "-1". In the weak guide field limit, the dominant acceleration mechanism is the Fermi process accomplished through the curvature drift motion of particles in magnetic flux tubes along the electric field induced by fast plasma flows. Simulations in the nonrelativistic regime give a softer spectrum. We present an analytical model for the formation of power-law distribution that is consistent with the analysis of the kinetic simulations. We further show that the acceleration of particles in kinetic simulations can be quantified by the energization from the fluid compression. Based on that, we suggest a new approach to model particle acceleration in large-scale magnetic reconnection in solar flare and other astrophysical reconnection regions.